A combined cycle power plant is a power plant configured by combining a gas turbine plant, a heat recovery steam generator (HRSG: Heat Recovery Steam Generator), and a steam turbine plant. In order to recover heat of exhaust gas of a gas turbine, the heat recovery steam generator incorporates a heat exchanger such as a superheater or a reheater. Here, the reheater of the heat recovery steam generator is a tube (heat transfer tube) such as the reheater representatively, and other than that, is a general term of the one composed of various components such as a header, connecting piping or the like, and is called the reheater hereinafter for simplification of description.
A role of the reheater is to generate reheat steam by heat-exchanging discharge steam which is exhaust of a high pressure steam turbine with gas turbine exhaust gas and overheating it. On the other hand, the reheater itself is cooled by the discharge steam passing through the inside. As a result, a temperature of the reheaters is settled, that is, balanced near a temperature of the reheat steam that passes through the inside. Here, the reheat steam becomes the highest temperature generally at an outer side surface part to be in direct contact with the gas turbine exhaust gas.
Then, the highest using temperature of the reheater is selected by imparting a necessary and sufficient margin in consideration of a gas turbine exhaust gas temperature, a flow rate of the reheat steam which is a cooling fluid or the like for an assumed plant operation. In recent examples, the highest using temperature of the reheater is, in many cases, turned to about 550° C. to 600° C. in a heat recovery steam generator combined with a gas turbine having a characteristic that the highest temperature of the gas turbine exhaust gas temperature is 600° C. to 650° C. Then, a plant operation accompanied by the gas turbine exhaust gas temperature exceeding the highest using temperature of the reheater is allowed under a condition that a cooling effect of the reheat steam is demonstrated, under a guarantee of a reheat steam flow rate in particular. A concept of cooling relating to the reheater is similar for the superheater which is the same heat exchanger, and the superheater is cooled by main steam that passes through the superheater.